Air conditioning systems in motor vehicles are subject to coolant leakage and require periodic refills. It is therefore necessary to carry out a system recharge and/or regeneration; to this end, devices have been developed for maintenance of air conditioning systems able to carry out the recovery, recycling and recharging of the coolant inside the system itself.
In the charging techniques of air conditioning systems for motor vehicles, the control of the so-called “incondensable” gases, i.e. gases which cannot be liquefied at ordinary temperature and pressure, takes on considerable importance. In practice this is air whose presence, in solution (in liquid or vapor phase) with the coolant gases (R-134a and R-1234yf are those currently universally used by the industry) causes some drawbacks described below.
As seen in FIG. 1, the presence of incondensable gases increases the liquid-vapor equilibrium pressure of the coolant: in fact, this pressure in the pure coolant is equal to the vapor pressure of the coolant itself and increases with increasing concentration of incondensable gas according to Dalton's law. The pressure rise in the A/C system in operation translates into a reduction of the COP of the compressor, i.e. a reduction of yield and thus a higher fuel consumption: intuitively, this is because air participates very marginally in the refrigerating cycle, as it does not change state during a cycle of an A/C system for automotive use, and because the compressor must compress the coolant-air mixture at higher pressures than it would with pure coolant.
The amounts of air are still very limited: on average, they are parts per million (ppm), since the air enters just as undesired component through repeated connection/disconnection cycles of the service fittings of the A/C system. In fact, every A/C system for automotive use is provided with two connectors, as will apparent from the following description, called service ports: one for the high pressure (HP) side and one for the low pressure (LP) side. Such ports, and the corresponding fittings supplied to the recharging stations, are regulated by the SAE J3639 standard.
During normal use, a dead space remains for each fitting of the quick coupling on the corresponding port in which the ambient air is trapped and then recovered by the station through the coolant. Typically, there are 2 cubic centimeters of air (about 2 milligrams) for systems with a capacity of about 500 grams, i.e. 4 ppm. This amount of air remains mostly dissolved in the vapor phase of the storage cylinder of the charging station; since the A/C system charging takes place by taking coolant from the liquid phase in the storage cylinder, most of the air is not returned to the system. A progressive accumulation of air thus occurs, to the point that a significant fraction may also be present in the liquid phase, thus “polluting” the A/C system.
In addition to the above, it should also be noted that one or more incorrect operations by the operator of the charging system may cause an accidental suction of ambient air by the station.
For this reason, the SAE J2099 standard details some requirements of purity of the coolant used for charging: the charging stations must ensure that the gas used for charging contains not more than 150 ppm air by weight.
The prior art used to monitor the presence of incondensable gases involves the measurement of temperature T and pressure P of the storage cylinder inside the climate station. When the station is inactive (but obviously switched on, so that the control electronics can implement the algorithm described hereinafter), the software checks that the coordinate pair (P, T) is on the liquid-vapor equilibrium line of the pure coolant (shown in FIG. 3). If the measured pressure is higher than what it should be at temperature T, there are incondensable gases and through a special valve on top of the cylinder, the same is “purged”, i.e. a vapor phase that (at least in theory) consists primarily of air (a stratification occurs, since air is lighter than the coolant vapor, as shown in FIG. 5) is vented.
Said technique has the main drawback of requiring a dedicated pressure sensor on the cylinder, which is generally expensive and, more importantly, is used only during “maintenance” operations of the coolant: typically, the need for air bleeding occurs once every two weeks, rarely more often.